If you have minor children, your estate plan should address who you will nominate as guardians for your children in the event you are no longer to care for them. If you pass away or are incapacitated, you want to make sure your plan addresses who will be in charge of your kids. In your estate plan, you do so by "nominating" a guardian.
So where do you nominate guardians for your kids? Generally speaking, the nomination of guardian(s) is found in your last will and testament. Also, you can also nominate guardians in a separate, stand alone document. Remember to select "primary" and "secondary" guardians for your children in case your first choices are unable to serve.
Just because you name someone doesn't mean they are automatically legal guardian over your children. Many people are surprised to learn that a nomination is simply just that - a nomination. The person you've chosen to act as guardian still needs to be appointed by the court. This is called a "Petition for Guardianship." Once the court approves your nomination, they are officially the legal guardian. The only way your nominee for guardian can become legal guardian is to be appointed by a court. A letter you write and give to them doesn't really give them any legal authority.
Therefore, armed with your nomination, your nominee must "petition" the court for guardianship. Then the judge will look at any specific instructions you may have put in your nomination, and may also make rulings regarding your wishes. The judge may not approve everything you request, but consider your nomination the place where you can at least make your general wishes and desires known. The court will consider your nomination but will also consider other factors affecting the welfare of your child. The latter is the overriding concern with strong consideration given to your nominations. Your children may also have a say if they are old enough.
Once appointed by a court, the guardian must generally regularly report to the court regularly about how the guardianship is going, provide an accounting, etc. If there are concerns, the court basically retains jurisdiction over the guardianship until the minor reaches age of majority. If a guardian is not doing a good job, they can be taken to court and questioned. Therefore, nominating a guardian or guardians for your minor children is a very important first step. Just make sure your nominees understand they must petition the court to officially be appointed guardian.